The invention relates generally to leak detection apparatus and more particularly, to automated leak detection apparatus for automatically handling and testing objects.
Many devices contain areas which must be kept isolated from other areas in the same device, or from the external environment. In the field of vehicle manufacturing for example, devices such as drum wheels, pumps, housings, cylinder blocks and cylinder heads must be kept relatively leak free. One common requirement of such devices is that they are free of structural flaws that could result in leakage of a fluid such as air or any of various corrosive or flammable solutions contained elsewhere in the device or outside the device. In the case of a wheel drum, certain minute structural flaws in the drum may result in very slow leakage of the pressurized air in the tire through the drum, evidence of which may not appear until after the wheel has been in use for some time.
Several methods have been used in the past for detecting leaks. For example, in the case of a drum wheel fitted with an inflated tire, the tire would be inflated with a probe gas which was preferably a mixture of air and helium. In one embodiment, the entire wheel assembly was enclosed within a heat shrinkable plastic envelope which formed a seal around the article under test. Any volume of probe gas (helium) which leaked through the valve stem or through the rim of the drum wheel was contained within the plastic envelope. Subsequently, the envelope was punctured with a conventional helium detector to test the space within the chamber formed by the plastic envelope for the presence of helium.
The use of the disposable plastic envelope to provide a seal tended to decrease the reproducibility of the results. The non-reproducibility of the results together with the tire employed to contain the probe gas made automated testing difficult. Also, this particular method and apparatus were concerned primarily with detecting leakage in the rim-tire and the valve stem interfaces and not through the drum wheel structure itself.
Another technique for detecting leaks in flexible articles employed a method typically referred to as leak detection by vacuum. The procedure involved leak detection from the outside to the inside of articles having easily deformable walls and was generally performed on an article which was mounted within a dome. Initially, a vacuum was drawn in a space of the article through a nipple which communicated with a test gas detector. The article was sprayed with a test gas and when a leak was present, the test gas penetrated the evacuated space. The procedure employed a disposable hood comprising plastic and was utilized to contain a gas mixture or to trap an escaping probe gas. Such a procedure also was difficult to automate.
A prior technique commonly known as the water immersion test and shown in FIG. 1 was employed for testing drum wheels for structural integrity and leakage of a pneumatic medium. A pneumatic test was conducted by clamping the wheel 50 under test between two platens 52, 54 along the axial center line of the wheel 50. The two platens each had elastomeric engaging disks 56, 58 on opposite interior faces for providing engaging and sealing surfaces. Upon capturing a drum wheel, a chamber 60 was defined within the wheel cavity and between the two opposing elastomeric engaging disks. A compressive force was applied to the opposing platens for creating a seal. The chamber 60 formed within the wheel cavity was then pressurized by means such as pneumatic hose 62 and the wheel was submerged in a water bath 64 inside a tank 66 and visually inspected for the formation of air bubbles; the presence of bubbles indicating the passage of air from the interior defined chamber to the outer exposed wheel surface. The formation and release of bubbles provided information on estimating the leak rate of the article being tested. Often, the article being tested was inspected through a porthole positioned in each of the walls of the tank and some test devices of the past also included various means for rotating the wheel to facilitate the search for air bubbles.
This technique suffered from numerous deficiencies. In one case, a slow leak through the structure of the wheel tested resulted in an air bubble clinging to the surface of the wheel. Under these conditions, the inspector had to discern a bubble originating from air leakage from a bubble caused by the mere immersion of the wheel in the bath, a subjective test at best. Initially, the procedure required the inspector to determine whether the bubble "broke away" from the wheel structure within a particular time limit. If the bubble did not "break away" within the prescribed time limit, the wheel passed the test. This subjective inspection by a human observer was found to be unsatisfactory.
Another particular deficiency in this test is that it was entirely possible that several microscopic holes or passages existed in the wheel. During the water immersion test, no single microscopic hole produced a bubble forming on the side of the wheel. However, the aggregate sum of the several microscopic holes could produce significant leakage over time. Also, it should be noted that the air pressure was applied from within the inner chamber formed by the two platens capturing the drum wheel. This pressurized force was applied in a direction opposite to that which would be experienced by the wheel under normal use. Under normal conditions with an inflated tire mounted on a wheel, the air pressure was applied from the external side between the tire and the external surface of the wheel. In an improperly casted wheel, the microscopic flaws in the cast aluminum may cause the wheel to pass the pressurized medium from the external surface of the wheel to the internal surface but not from the internal surface to the external surface of the wheel. This is because imperfections such as minute microscopic aluminum flakes may tend to function as miniature check valves permitting the air pressure to pass in one direction but not in the opposite direction.
Another testing system which is disclosed in U.S. Pat. No. 4,813,268 to Helvey employed a top and a bottom platen to provide an inner chamber within the structure of the wheel. A housing was then lowered over the wheel to form an outer chamber which enclosed the wheel. A partial vacuum was then drawn on the inner chamber and a test gas such as helium was introduced into the outer chamber. If leakage existed in the structure of the wheel, the helium-air mixture would pass from the outer chamber to the lower-pressure inner chamber. A helium monitoring device sampled the inner chamber and detected the presence of the test gas if a leak existed.
While the Helvey patent represents a significant advance in the art, a substantial amount of time had to be spent to purge the test gas from the system after testing and this process tended to slow the testing process. Helvey also included no means for automatically measuring the size of the object under test and comparing the test results to the stored data for objects of that particular size to determine whether the object passed or failed the test. Helvey also had no automated handling means for moving objects through the automated test procedure. Many devices, for example drum wheels, require one hundred percent testing, thus efficiency and speed in the testing process is necessary to maintain production rates. Automating the testing process and improving the process itself so that faster purging of the test gas occurs, and so that differently sized articles can be tested without reconfiguring the test apparatus would enable higher production rates.
It would therefore be an advance in the art to provide a leak detection system which improves the production testing rate, which can test objects of differing sizes and styles without reconfiguring the test apparatus, which has an improved test gas purging system, which permits automatic identification and rejection of structures that fail the test, and which provides self-testing and self-calibrating procedures for preventing faulty rejection of good objects and acceptance of bad objects.